Wow! Here's and odd one I just read about…
It's a new Decision added to the Rules this year.
Not sure I agree with this one, but here it is:

14-3/17Player Listens to Music or Broadcast During Round

Q.A player uses a device to listen to music, a radio broadcast or any other type of broadcast during a stipulated round. What is the ruling?

A.Under Rule 14-3a, a player may not use any artificial device or unusual equipment that "might assist him in making a stroke or in his play." Listening to music or a broadcast while making a stroke or for a prolonged period might assist the player in his play, for example, by eliminating distractions or promoting a good tempo. Therefore, the use of an artificial device to listen to music or a broadcast, whether or not through headphones, while making a stroke or for a prolonged period of time during a stipulated round is a breach of Rule 14-3. However, it would not be a breach of Rule 14-3 for a player to listen to a device briefly, for example, to obtain the results of another sporting event or traffic information, while walking between the putting green of one hole and the teeing ground of the next hole.

A Committee will have to consider all available facts and circumstances in determining whether a player using an artificial device to listen to music or a broadcast has done so for a prolonged period such that the action might have assisted the player in his play.

There is no restriction on listening to music or other broadcasts while practicing (whether on the practice ground or on the golf course, and whether by oneself or while playing with others), although club rules and disciplinary codes could apply in such circumstances. (New)

Other Decisions related to Rule 14-3: See "Artificial Devices, Unusual Equipment and Unusual Use of Equipment" in the Index.